U.S. Pat. No. 7,131,926B (hereinafter referred to Patent reference 1) discloses a known automatic transmission which includes three single pinion type planetary gear mechanisms and six engaging elements including two brakes and four clutches that selectively engage and disengage elements of the planetary gear mechanisms. The known automatic transmission disclosed in Patent reference 1 establishes nine forward speeds and one reverse speed by selectively engaging and disengaging the mentioned six engaging elements.
According to the automatic transmission disclosed in Patent reference 1, when a reverse gear stage is established, a sun gear of a third planetary gear set rotates in a reverse direction because a ring gear of the third planetary gear set is fixed to a housing by means of a brake and a planetary carrier provided at the third planetary gear set and connected to an output shaft rotates in a reverse direction. Whereas a ring gear provided at a first planetary gear set and connected to the sun gear of the third planetary gear set rotates in a reverse direction, because a planetary carrier provided at the first planetary gear set and connected to an input shaft rotates in a normal direction, a sun gear of the first planetary gear set rotates in high speed. Thus, durability of a member, for example, a bearing that supports the sun gear of the first planetary gear set is deteriorated. Further, because a relative rotation speed of a brake that is connected to the sun gear of the first planetary gear set is increased, the brake may be damaged, for example, burned. Still further, in a case where an oil passage is formed on a shaft that is directly connected to the sun gear of the first planetary gear set, durability of a seal ring that is in contact with the shaft is deteriorated.
A need thus exists for an automatic transmission which is not susceptible to the drawback mentioned above.